diff --git a/Nef_3/include/CGAL/Nef_3/SNC_io_parser.h b/Nef_3/include/CGAL/Nef_3/SNC_io_parser.h
index 3e4fea327ef..cd4fec99650 100644
--- a/Nef_3/include/CGAL/Nef_3/SNC_io_parser.h
+++ b/Nef_3/include/CGAL/Nef_3/SNC_io_parser.h
@@ -855,7 +855,7 @@ public:
     return this->string("unknown object");
   }
 
-  bool check_sep(char* sep) const;
+  bool check_sep(const char* sep) const;
   bool test_string(std::string s) const;
   void print_vertex(Vertex_handle) const;
   void print_edge(Halfedge_handle) const;
@@ -1114,7 +1114,7 @@ SNC_io_parser<EW>::SNC_io_parser(std::ostream& os, SNC_structure& W,
 }
 
 template <typename EW>
-bool SNC_io_parser<EW>::check_sep(char* sep) const
+bool SNC_io_parser<EW>::check_sep(const char* sep) const
 {
   char c; 
   do in.get(c); while (isspace(c));
diff --git a/Nef_3/include/CGAL/Nef_3/Shell_constructor.h b/Nef_3/include/CGAL/Nef_3/Shell_constructor.h
index 883513e5866..43e12299c21 100644
--- a/Nef_3/include/CGAL/Nef_3/Shell_constructor.h
+++ b/Nef_3/include/CGAL/Nef_3/Shell_constructor.h
@@ -38,7 +38,7 @@ class Shell_constructor : public Modifier_base<typename Nef3::SNC_structure> {
   std::istream in;
 
 
-  bool SNC_io_parser<EW>::check_sep(char* sep) const {
+  bool SNC_io_parser<EW>::check_sep(const char* sep) const {
     char c; 
     do in.get(c); while (isspace(c));
     while (*sep != '\0') { 
diff --git a/Nef_S2/include/CGAL/Nef_S2/SM_io_parser.h b/Nef_S2/include/CGAL/Nef_S2/SM_io_parser.h
index 8da1a1c4912..70f1dfc782d 100644
--- a/Nef_S2/include/CGAL/Nef_S2/SM_io_parser.h
+++ b/Nef_S2/include/CGAL/Nef_S2/SM_io_parser.h
@@ -83,7 +83,7 @@ class SM_io_parser : public Decorator_
   int vn,en,ln,fn,i;
   // the number of objects
 
-  bool check_sep(char* sep);
+  bool check_sep(const char* sep);
   void print_vertex(SVertex_handle) const;
   void print_edge(SHalfedge_handle) const;
   void print_loop(SHalfloop_const_handle) const;
@@ -161,7 +161,7 @@ SM_io_parser(std::ostream& iout, const Base& D)
 //-----------------------------------------------------------------------------
 
 template <typename Decorator_>
-bool SM_io_parser<Decorator_>::check_sep(char* sep)
+bool SM_io_parser<Decorator_>::check_sep(const char* sep)
 {
   char c; 
   do in.get(c); while (isspace(c));