The environs of three supernova remnants (SNRs) with associated OH 1720 MHz
masers, G349.7+0.2, CTB 37A, and G16.7+0.1, have been surveyed in the CO J
= 1-0 transition with the 12 Meter Telescope of the NRAO, using the on-the
-fly technique. These observations have revealed a number of molecular clou
ds interacting with the SNR shock fronts. Most of the OH 1720 MHz masers ha
ve been found to lie over CO concentrations, and the maser velocities are c
oincident with the CO peak velocities to an accuracy better than 2 km s(-1)
. The present data trace the interstellar medium (ISM) structures interacti
ng with the SNRs; however, to probe the shocked molecular gas in which the
OH 1720 MHz emission originates, higher excitation transitions and more com
plex species should be observed. In CTB 37A, where the shock velocity into
the molecular cloud could be determined, it has been found to be of C-type,
in agreement with theoretical predictions. Part of the rim of G16.7+0.1 ap
pears to be flattened by a dense external cloud, yet the only associated OH
1720 MHz maser lies near the opposite region of the remnant. This behavior
, also observed in IC 443 and 3C 391, seems to contradict the suggestion th
at OH 1720 MHz maser emission occurs mainly for transverse shocks.