Index: trunk/ppStack/src/ppStackArguments.c
===================================================================
--- trunk/ppStack/src/ppStackArguments.c	(revision 20494)
+++ trunk/ppStack/src/ppStackArguments.c	(revision 20497)
@@ -145,4 +145,6 @@
     psMetadataAddF32(arguments, PS_LIST_TAIL, "-combine-rej", 0,
                      "Combination rejection thresold (sigma)", NAN);
+    psMetadataAddF32(arguments, PS_LIST_TAIL, "-combine-sys", 0,
+                     "Relative systematic error in combination", NAN);
     psMetadataAddStr(arguments, PS_LIST_TAIL, "-mask-val", 0, "Mask value of input bad pixels", NULL);
     psMetadataAddStr(arguments, PS_LIST_TAIL, "-mask-bad", 0, "Mask value to give bad pixels", NULL);
@@ -226,4 +228,5 @@
     VALUE_ARG_RECIPE_INT("-iter",             "ITER",           S32, 0);
     VALUE_ARG_RECIPE_FLOAT("-combine-rej",    "COMBINE.REJ",    F32);
+    VALUE_ARG_RECIPE_FLOAT("-combine-sys",    "COMBINE.SYS",    F32);
     VALUE_ARG_RECIPE_FLOAT("-threshold-mask", "THRESHOLD.MASK", F32);
     VALUE_ARG_RECIPE_FLOAT("-image-rej",      "IMAGE.REJ",      F32);
Index: trunk/ppStack/src/ppStackReadout.c
===================================================================
--- trunk/ppStack/src/ppStackReadout.c	(revision 20494)
+++ trunk/ppStack/src/ppStackReadout.c	(revision 20497)
@@ -96,4 +96,5 @@
     int iter = psMetadataLookupS32(NULL, recipe, "ITER"); // Rejection iterations
     float combineRej = psMetadataLookupF32(NULL, recipe, "COMBINE.REJ"); // Combination threshold
+    float combineSys = psMetadataLookupF32(NULL, recipe, "COMBINE.SYS"); // Combination systematic error
     bool useVariance = psMetadataLookupBool(&mdok, recipe, "VARIANCE"); // Use variance for rejection?
     bool safe = psMetadataLookupBool(&mdok, recipe, "SAFE"); // Be safe when combining small numbers of pixels
@@ -133,6 +134,6 @@
     }
 
-    if (!pmStackCombine(outRO, stack, maskVal | maskBad, maskBad, kernelSize, iter, combineRej, true,
-                        useVariance, safe)) {
+    if (!pmStackCombine(outRO, stack, maskVal | maskBad, maskBad, kernelSize, iter, combineRej, combineSys,
+                        true, useVariance, safe)) {
         psError(PS_ERR_UNKNOWN, false, "Unable to combine input readouts with rejection.");
         psFree(stack);