  Very luminous extragalactic water masers, the megamasers, are associated with
active galactic nuclei (AGN) in galaxies characterized by accretion disks,
radio jets, and nuclear outflows. Weaker masers, the kilomasers, seem to be
mostly related to star formation activity, although the possibility exists that
some of these sources may belong to the weak tail of the AGN maser
distribution. It is of particular importance to accurately locate the water
maser emission to reveal its origin and shed light onto extragalactic star
forming activity or to elucidate the highly obscured central regions of
galaxies. We performed interferometric observations of three galaxies, NGC3556,
Arp299, and NGC4151, where water emission was found. Statistical tools have
been used to study the relation between OH and water maser emission in
galaxies. The maser in NGC3556 is associated with a compact radio continuum
source that is most likely a supernova remnant or radio supernova. In Arp299,
the luminous water maser has been decomposed in three main emitting regions
associated with the nuclear regions of the two main galaxies of the system,
NGC3690 and IC694, and the region of overlap. In NGC4151, only one of the two
previously observed maser components has been tentatively detected. This
feature, if real, is associated with the galaxy's central region. The only
galaxy, so far, where luminous maser emission from two maser species, OH and
H2O has been confidently detected is Arp299. Weaker masers from these two
species do instead coexist in a number of objects. A larger number of objects
searched for both maser species are, however, necessary to better assess these
last two results.
