diff --git a/CMakeLists.txt b/CMakeLists.txt
index 8dd2f63d..c5b00558 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -5,7 +5,7 @@ if (CMAKE_VERSION GREATER_EQUAL 3.12)
   cmake_policy(SET CMP0075 NEW)
 endif (CMAKE_VERSION GREATER_EQUAL 3.12)
 
-project(musrfit VERSION 1.7.3 LANGUAGES C CXX)
+project(musrfit VERSION 1.7.4 LANGUAGES C CXX)
 
 #--- musrfit specific options -------------------------------------------------
 option(nexus "build optional NeXus support. Needed for ISIS" OFF)
diff --git a/ChangeLog b/ChangeLog
index 7eea47d0..b511b1c5 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -12,11 +12,16 @@ or
 
 https://bitbucket.org/muonspin/musrfit/commits/all
 
+Release of V1.7.4, 2021/06/16
+=============================
+
+allow in asymmetry fits #forward groups != #backward groups
+
 Release of V1.7.3, 2021/06/01
 =============================
 
 making musrfit Apple Silicon M1 ready. Currently there is still an open issue 
-with NeXus.
+with NeXus (HDF4 only).
 
 Release of V1.7.2, 2021/04/16
 =============================
diff --git a/src/classes/PMsrHandler.cpp b/src/classes/PMsrHandler.cpp
index b40ae3db..4f246f61 100644
--- a/src/classes/PMsrHandler.cpp
+++ b/src/classes/PMsrHandler.cpp
@@ -6334,16 +6334,7 @@ Bool_t PMsrHandler::CheckHistoGrouping()
 {
   Bool_t result = true;
 
-  for (UInt_t i=0; i<fRuns.size(); i++) {
-    if (fRuns[i].GetFitType() == MSR_FITTYPE_ASYM || fRuns[i].GetFitType() == MSR_FITTYPE_BNMR) {
-      if (fRuns[i].GetForwardHistoNoSize() != fRuns[i].GetBackwardHistoNoSize()) {
-        std::cerr << std::endl << ">> PMsrHandler::CheckHistoGrouping: **ERROR** # of forward histos != # of backward histos.";
-        std::cerr << std::endl << ">> Run #" << i+1;
-        std::cerr << std::endl;
-        result = false;
-        break;
-      }
-    }
+  for (UInt_t i=0; i<fRuns.size(); i++) {    
     // check grouping entries are not identical, e.g. forward 1 1 2
     if (fRuns[i].GetForwardHistoNoSize() > 1) {
       for (UInt_t j=0; j<fRuns[i].GetForwardHistoNoSize(); j++) {
diff --git a/src/classes/PRunAsymmetry.cpp b/src/classes/PRunAsymmetry.cpp
index 357672ae..2dfad30a 100644
--- a/src/classes/PRunAsymmetry.cpp
+++ b/src/classes/PRunAsymmetry.cpp
@@ -602,17 +602,7 @@ Bool_t PRunAsymmetry::PrepareData()
       return false;
     }
   }
-  if (forwardHistoNo.size() != backwardHistoNo.size()) {
-    std::cerr << std::endl << ">> PRunAsymmetry::PrepareData(): **PANIC ERROR**:";
-    std::cerr << std::endl << ">> # of forward histograms different from # of backward histograms.";
-    std::cerr << std::endl << ">> Will quit :-(";
-    std::cerr << std::endl;
-    // clean up
-    forwardHistoNo.clear();
-    backwardHistoNo.clear();
-    return false;
-  }
-
+  
   // keep the time resolution in (us)
   fTimeResolution = runData->GetTimeResolution()/1.0e3;
   std::cout.precision(10);
@@ -626,11 +616,13 @@ Bool_t PRunAsymmetry::PrepareData()
   // keep the histo of each group at this point (addruns handled below)
   std::vector<PDoubleVector> forward, backward;
   forward.resize(forwardHistoNo.size());   // resize to number of groups
-  backward.resize(backwardHistoNo.size()); // resize to numer of groups
   for (UInt_t i=0; i<forwardHistoNo.size(); i++) {
     forward[i].resize(runData->GetDataBin(forwardHistoNo[i])->size());
-    backward[i].resize(runData->GetDataBin(backwardHistoNo[i])->size());
     forward[i]  = *runData->GetDataBin(forwardHistoNo[i]);
+  }
+  backward.resize(backwardHistoNo.size()); // resize to number of groups
+  for (UInt_t i=0; i<backwardHistoNo.size(); i++) {
+    backward[i].resize(runData->GetDataBin(backwardHistoNo[i])->size());
     backward[i] = *runData->GetDataBin(backwardHistoNo[i]);
   }
 
@@ -676,9 +668,11 @@ Bool_t PRunAsymmetry::PrepareData()
 
   // set forward/backward histo data of the first group
   fForward.resize(forward[0].size());
-  fBackward.resize(backward[0].size());
   for (UInt_t i=0; i<fForward.size(); i++) {
     fForward[i]  = forward[0][i];
+  }
+  fBackward.resize(backward[0].size());
+  for (UInt_t i=0; i<fBackward.size(); i++) {
     fBackward[i] = backward[0][i];
   }
 
@@ -1677,7 +1671,7 @@ Bool_t PRunAsymmetry::PrepareRRFViewData(PRawRunData* runData, UInt_t histoNo[2]
 //--------------------------------------------------------------------------
 /**
  * <p>Get the proper t0 for the single histogram run.
- * -# the t0 vector size = number of detectors (grouping) for forward.
+ * -# the t0 vector size = number of detectors (grouping) for forward + backward.
  * -# initialize t0's with -1
  * -# fill t0's from RUN block
  * -# if t0's are missing (i.e. t0 == -1), try to fill from the GLOBAL block.
@@ -1698,7 +1692,11 @@ Bool_t PRunAsymmetry::GetProperT0(PRawRunData* runData, PMsrGlobalBlock *globalB
   // feed all T0's
   // first init T0's, T0's are stored as (forward T0, backward T0, etc.)
   fT0s.clear();
-  fT0s.resize(2*forwardHistoNo.size());
+  // this strange fT0 size estimate is needed in case #forw histos != #back histos
+  size_t size = 2*forwardHistoNo.size();
+  if (backwardHistoNo.size() > forwardHistoNo.size())
+    size = 2*backwardHistoNo.size();
+  fT0s.resize(size);
   for (UInt_t i=0; i<fT0s.size(); i++) {
     fT0s[i] = -1.0;
   }
@@ -1765,6 +1763,8 @@ Bool_t PRunAsymmetry::GetProperT0(PRawRunData* runData, PMsrGlobalBlock *globalB
       std::cerr << std::endl;
       return false;
     }
+  }
+  for (UInt_t i=0; i<backwardHistoNo.size(); i++) {
     if ((fT0s[2*i+1] < 0) || (fT0s[2*i+1] > static_cast<Int_t>(runData->GetDataBin(backwardHistoNo[i])->size()))) {
       std::cerr << std::endl << ">> PRunAsymmetry::PrepareData(): **ERROR** t0 data bin (" << fT0s[2*i+1] << ") doesn't make any sense!";
       std::cerr << std::endl << ">> backwardHistoNo " << backwardHistoNo[i];
diff --git a/src/classes/PRunAsymmetryBNMR.cpp b/src/classes/PRunAsymmetryBNMR.cpp
index 7d533e6d..7946717e 100644
--- a/src/classes/PRunAsymmetryBNMR.cpp
+++ b/src/classes/PRunAsymmetryBNMR.cpp
@@ -640,6 +640,7 @@ Bool_t PRunAsymmetryBNMR::PrepareData()
       return false;
     }
   }
+/* //as35
   if (forwardHistoNo.size() != backwardHistoNo.size()) {
     std::cerr << std::endl << ">> PRunAsymmetryBNMR::PrepareData(): **PANIC ERROR**:";
     std::cerr << std::endl << ">> # of forward histograms different from # of backward histograms.";
@@ -650,7 +651,8 @@ Bool_t PRunAsymmetryBNMR::PrepareData()
     backwardHistoNo.clear();
     return false;
   }
-
+*/ //as35
+  
   // keep the time resolution in (s)
   fTimeResolution = runData->GetTimeResolution()/1.0e3;
   std::cout.precision(10);
@@ -664,11 +666,13 @@ Bool_t PRunAsymmetryBNMR::PrepareData()
   // keep the histo of each group at this point (addruns handled below)
   std::vector<PDoubleVector> forward, backward;
   forward.resize(forwardHistoNo.size());   // resize to number of groups
-  backward.resize(backwardHistoNo.size()); // resize to numer of groups
   for (UInt_t i=0; i<forwardHistoNo.size(); i++) {
     forward[i].resize(runData->GetDataBin(forwardHistoNo[i])->size());
-    backward[i].resize(runData->GetDataBin(backwardHistoNo[i])->size());
     forward[i]  = *runData->GetDataBin(forwardHistoNo[i]);
+  }
+  backward.resize(backwardHistoNo.size()); // resize to number of groups
+  for (UInt_t i=0; i<backwardHistoNo.size(); i++) {
+    backward[i].resize(runData->GetDataBin(backwardHistoNo[i])->size());
     backward[i] = *runData->GetDataBin(backwardHistoNo[i]);
   }
 
@@ -712,15 +716,18 @@ Bool_t PRunAsymmetryBNMR::PrepareData()
     }
   }
 
-  // set forward/backward histo data of the first group
+  // set forward histo data of the first group
   fForwardp.resize(forward[0].size());
-  fBackwardp.resize(backward[0].size());
   fForwardm.resize(forward[0].size());
-  fBackwardm.resize(backward[0].size());
   for (UInt_t i=0; i<fForwardp.size(); i++) {
     fForwardp[i]  = forward[0][i];
-    fBackwardp[i] = backward[0][i];
     fForwardm[i]  = forward[1][i];
+  }
+  // set backward histo data of the first group
+  fBackwardp.resize(backward[0].size());
+  fBackwardm.resize(backward[0].size());
+  for (UInt_t i=0; i<fBackwardp.size(); i++) {
+    fBackwardp[i] = backward[0][i];
     fBackwardm[i] = backward[1][i];
   }
 
@@ -1529,7 +1536,11 @@ Bool_t PRunAsymmetryBNMR::GetProperT0(PRawRunData* runData, PMsrGlobalBlock *glo
   // feed all T0's
   // first init T0's, T0's are stored as (forward T0, backward T0, etc.)
   fT0s.clear();
-  fT0s.resize(2*forwardHistoNo.size());
+  // this strange fT0 size estimate is needed in case #forw histos != #back histos
+  size_t size = 2*forwardHistoNo.size();
+  if (backwardHistoNo.size() > forwardHistoNo.size())
+    size = 2*backwardHistoNo.size();
+  fT0s.resize(size);
   for (UInt_t i=0; i<fT0s.size(); i++) {
     fT0s[i] = -1.0;
   }
@@ -1596,6 +1607,8 @@ Bool_t PRunAsymmetryBNMR::GetProperT0(PRawRunData* runData, PMsrGlobalBlock *glo
       std::cerr << std::endl;
       return false;
     }
+  }
+  for (UInt_t i=0; i<backwardHistoNo.size(); i++) {
     if ((fT0s[2*i+1] < 0) || (fT0s[2*i+1] > static_cast<Int_t>(runData->GetDataBin(backwardHistoNo[i])->size()))) {
       std::cerr << std::endl << ">> PRunAsymmetryBNMR::PrepareData(): **ERROR** t0 data bin (" << fT0s[2*i+1] << ") doesn't make any sense!";
       std::cerr << std::endl << ">> backwardHistoNo " << backwardHistoNo[i];
diff --git a/src/classes/PRunAsymmetryRRF.cpp b/src/classes/PRunAsymmetryRRF.cpp
index 125188c4..1d83808c 100644
--- a/src/classes/PRunAsymmetryRRF.cpp
+++ b/src/classes/PRunAsymmetryRRF.cpp
@@ -584,16 +584,6 @@ Bool_t PRunAsymmetryRRF::PrepareData()
       return false;
     }
   }
-  if (forwardHistoNo.size() != backwardHistoNo.size()) {
-    std::cerr << std::endl << ">> PRunAsymmetryRRF::PrepareData(): **PANIC ERROR**:";
-    std::cerr << std::endl << ">> # of forward histograms different from # of backward histograms.";
-    std::cerr << std::endl << ">> Will quit :-(";
-    std::cerr << std::endl;
-    // clean up
-    forwardHistoNo.clear();
-    backwardHistoNo.clear();
-    return false;
-  }
 
   // keep the time resolution in (us)
   fTimeResolution = runData->GetTimeResolution()/1.0e3;
@@ -608,11 +598,13 @@ Bool_t PRunAsymmetryRRF::PrepareData()
   // keep the histo of each group at this point (addruns handled below)
   std::vector<PDoubleVector> forward, backward;
   forward.resize(forwardHistoNo.size());   // resize to number of groups
-  backward.resize(backwardHistoNo.size()); // resize to numer of groups
   for (UInt_t i=0; i<forwardHistoNo.size(); i++) {
     forward[i].resize(runData->GetDataBin(forwardHistoNo[i])->size());
-    backward[i].resize(runData->GetDataBin(backwardHistoNo[i])->size());
     forward[i]  = *runData->GetDataBin(forwardHistoNo[i]);
+  }
+  backward.resize(backwardHistoNo.size()); // resize to number of groups
+  for (UInt_t i=0; i<backwardHistoNo.size(); i++) {
+    backward[i].resize(runData->GetDataBin(backwardHistoNo[i])->size());
     backward[i] = *runData->GetDataBin(backwardHistoNo[i]);
   }
 
@@ -1281,7 +1273,11 @@ Bool_t PRunAsymmetryRRF::GetProperT0(PRawRunData* runData, PMsrGlobalBlock *glob
   // feed all T0's
   // first init T0's, T0's are stored as (forward T0, backward T0, etc.)
   fT0s.clear();
-  fT0s.resize(2*forwardHistoNo.size());
+  // this strange fT0 size estimate is needed in case #forw histos != #back histos
+  size_t size = 2*forwardHistoNo.size();
+  if (backwardHistoNo.size() > forwardHistoNo.size())
+    size = 2*backwardHistoNo.size();
+  fT0s.resize(size);
   for (UInt_t i=0; i<fT0s.size(); i++) {
     fT0s[i] = -1.0;
   }
@@ -1348,6 +1344,8 @@ Bool_t PRunAsymmetryRRF::GetProperT0(PRawRunData* runData, PMsrGlobalBlock *glob
       std::cerr << std::endl;
       return false;
     }
+  }
+  for (UInt_t i=0; i<backwardHistoNo.size(); i++) {
     if ((fT0s[2*i+1] < 0) || (fT0s[2*i+1] > static_cast<Int_t>(runData->GetDataBin(backwardHistoNo[i])->size()))) {
       std::cerr << std::endl << ">> PRunAsymmetryRRF::PrepareData(): **ERROR** t0 data bin (" << fT0s[2*i+1] << ") doesn't make any sense!";
       std::cerr << std::endl << ">> backwardHistoNo " << backwardHistoNo[i];