The water megamaser in the merger system Arp 299

Abstract We present preliminary results of an interferometric study of the water megamaser in the merger system Arp 299. This system is composed of two main sources: IC 694 and NGC 3690. There is clear evidence that most of the water maser emission is associated with the nucleus of the latter, confirming the presence of an optically obscured AGN as previously suggested by X-ray observations. Furthermore, emission arises from the inner regions of IC 694, where an OH megamaser is also present. The velocity of the water maser line is blueshifted w.r.t. the optically determined systemic velocity and is consistent with that of the OH megamaser line. This finding might then indicate that both masers are associated with the same (expanding) structure and that, for the first time, strong 22 GHz H 2 O and 1.67 GHz OH maser emission has been found to coexist.