Rihanna -- I'm STILL RIDIN' with Chris Brown

Rihanna has once again taken back Chris Brown ... at least for now ... 'cause the two were rollin' around L.A. together in the same expensive car yesterday.

As we previously reported, the two have been breaking up and getting back together more often than Van Halen ... but yesterday, it seems they were "back on" for the day.

In fact, Rihanna even tweeted a pic of the twosome together looking as coupley as ever.

Think they'll make it through the week?