diff --git a/Makefile b/Makefile
index 865dcf6..c39dcc1 100644
--- a/Makefile
+++ b/Makefile
@@ -1,4 +1,5 @@
 # this file is under GNU GPL 3.0, see LICENSE.txt
+# see LICENSE.txt
 
 include Makefile.conf
 -include local.conf
diff --git a/Makefile.conf b/Makefile.conf
index e652e79..b409f75 100644
--- a/Makefile.conf
+++ b/Makefile.conf
@@ -1,3 +1,6 @@
+# this file is under GNU GPL 3.0, see LICENSE.txt
+# see LICENSE.txt
+
 TARGET=libpEpAdapter.a
 PREFIX=$(HOME)
 PEP=$(HOME)
diff --git a/test_adapter.cc b/test_adapter.cc
index 15f8d88..4f2241e 100644
--- a/test_adapter.cc
+++ b/test_adapter.cc
@@ -1,3 +1,6 @@
+// This file is under GNU General Public License 3.0
+// see LICENSE.txt
+
 #include "Adapter.hh"
 #include <iostream>
 #include <assert.h>