.net mvc send json with mixed types from controller

So I have this JsonResult:

[HttpPost]
public JsonResult getJsonInvoicesClient(int id_client)
{
    var a = db.invoices
        .Where(x => x.id_client == id_client)
        .ToList();
    return Json(a);
}

and in jquery I receive a list of invoice objects. Now I need to add the client data to the json, so I get it from the db with something like:

    var c = db.clients.FirstOrDefault(p => p.id == id_client);

But how can I add this client object to the json... so that in jquery I receive a list or similar with just 2 items: the client object, and the list of invoice objects?