Authentication or using session inter-project

I have a problem that I have could not solve yet.

that is problem;

I have a project written with VB.NET. And I'm using Forms authentication for
security. but now, I want to write new code (new modules) with c#. so I have
added new project to my solution. and then when I want to use forms
authentication I failed. New project is under my old project's folder.

I deleted web.config for new project then tried again but I failed.
I faced 2 problems;
first: login page couldn't return page which redirect itself.
second; after I logged on I write to path myself but login page appear again.

whatever I do I can't success.

can you help me? what can I do?

shortly; I have 2 projects. and they should use same login page same
session etc.

where did I make mistake?