diff --git a/TODO b/TODO
index 5cc732f3..b67d0a3e 100644
--- a/TODO
+++ b/TODO
@@ -1,3 +1,7 @@
+- started bandjoinconst
+
+  see also unaryconst.c
+
 - looks like we have a race in tiled threadcache? see 
 
 	https://github.com/jcupitt/libvips/issues/347
diff --git a/libvips/conversion/bandjoin.c b/libvips/conversion/bandjoin.c
index 6b616060..b04a332e 100644
--- a/libvips/conversion/bandjoin.c
+++ b/libvips/conversion/bandjoin.c
@@ -261,3 +261,22 @@ vips_bandjoin2( VipsImage *in1, VipsImage *in2, VipsImage **out, ... )
 
 	return( result );
 }
+
+typedef struct _VipsBandjoinConst {
+	VipsBandary parent_instance;
+
+	VipsImage *in;
+	VipsArrayDouble *c;
+
+	/* The constant expanded to in's format, ready to be appended to each
+	 * pixel.
+	 */
+	int n;
+	VipsPel *c_ready;
+
+} VipsBandjoinConst;
+
+typedef VipsBandaryClass VipsBandjoinConstClass;
+
+G_DEFINE_TYPE( VipsBandjoinConst, vips_bandjoin_const, VIPS_TYPE_BANDARY );
+