More questions on multiple apps, single sign-on

There has been some discussion on this, but I want to revive it a bit.

I'm going to have multiple Rails apps, each with their own subdomain in
a tld:

I want to have a single login work across applications. A given user
can have data in each application, but the data will be distinct. That
is, I don't see a lot of shared code between applications other than

There seem to be 2 choices: a) Separate app for login, then have an
app-per-subdomain redirect there and back or b) Multiple applications
using the same database.

Separate app for login