Login programmatically on register

function _login(&$form_state, &$edit)
{	
	$email = $form_state['account']['name']['#post']['mail'];
	$user = user_load(array('mail' => $email));
	user_external_login($user);
}
 
function custom_form_alter(&$form, $form_state, $form_id)
{
	if($form_id == "user_register"):
		$form['#submit'][] = '_login';
	endif;
 
	return $form;
}