The FIDE candidates matches will begin on the 26th of May in Elista. The first round will consist of eight matches over a distance of six games. You can see the original FIDE news item here. The most important facts about the match between Aronian and Carlsen can be found on the official site of the Mexico tournament here.
I will resume my previews of the Kramnik-Topalov match and have a look at the openings that we may see in the forthcoming candidate matches. These matches are played over a shorter distance - if one player is behind in the match he may have to risk something immediately. On the other hand they would not want to risk much as long as the score is even.
It is remarkable that Magnus Carlsen plays both 1.e4 and 1.d4 regularly at his young age. The ability to play both of these major opening moves has been regarded as important for match play at the highest level, but many world champion candidates started out their international carreer playing only one of those moves and learned the other later. For example Peter Leko had played 1.e4 in almost all of his games, but had prepared 1.d4 for his match against Kramnik.
Against 1.e4 Aronian plays the Marshall. The Anti-Marshall variations with 8.h3 are currently en-vogue on the top level, but personally I feel that while these systems are not so easy to play, they don't pose any theoretical danger for Black. In fact Aronian seems to be very comfortable in these positions with Black and scores very well there.
Carlsen must have noticed that and has started all his White games against Aronian with 1.d4 so far.
Against 1.d4 Aronian usually plays 1..Nf6 2.c4 e6, answering 3.Nc3 with the Nimzo, and 3.Nf3 with the Queens Indian or 3..d5. Carlsen has tried all of these systems with White, and sometimes the Catalan, and both have played different sub-variations of all of these openings, so it is a bit hard to guess what we will see - but unless Carlsen has some excellent preparation against the Marshall, I expect to see 1.d4 from Carlsen unless he is in the lead (because it is not so easy for Black to win with the Marshall either, especially if White plays one of the known forced drawing lines).
Aronian is a 1.d4 player and seems to play something else only in the Monaco event. Again, Carlsen plays the same openings after 1..Nf6 2.c4 e6 we just discussed with colours reversed, and Aronian also plays different systems against the Nimzo, Queens Indian, and has also tried the Catalan a couple of times recently.
Aronian sometimes finds amazing ways to play on in positions that look harmless for Black, so Carlsen has to be very careful. If Carlsen is behind in the match, expect him to play something sharper, like the King's Indian or the Volga gambit.
Both players have a very solid opening repertoire with Black. Unless we see some good novelties or some bad blunders, this match may well be decided in the tie breaks.