I found a solution on the Codex 🙂
This works 🙂
/**
* Registering Post Types
*/
// Show posts of 'post', 'page' and 'oeuvre' post types on category page
add_action( 'pre_get_posts', 'add_my_post_types_to_query' );
function add_my_post_types_to_query( $query ) {
if ( is_category() && $query->is_main_query() )
$query->set( 'post_type', array(
'post',
'page',
'oeuvre'
));
return $query;
}