We present results of a multi-epoch monitoring program on variability of 6 cm formaldehyde (H 2 CO) masers in the massive star forming region NGC 7538 IRS 1 from 2008 to 2015 conducted with the GBT, WSRT, and VLA. We found that the similar variability behaviors of the two formaldehyde maser velocity components in NGC 7538 IRS 1 (which was pointed out by Araya and collaborators in 2007) have continued. The possibility that the variability is caused by changes in the maser amplification path in regions with similar morphology and kinematics is discussed. We also observed 12.2 GHz methanol and 22.2 GHz water masers toward NGC 7538 IRS 1. The brightest maser components of CH 3 OH and H 2 O species show a decrease in flux density as a function of time. The brightest H 2 CO maser component also shows a decrease in flux density and has a similar LSR velocity to the brightest H 2 O and 12.2 GHz CH 3 OH masers. The line parameters of radio recombination lines and the 20.17 and 20.97 GHz CH 3 OH transitions in NGC 7538 IRS 1 are also reported. In addition, we observed five other 6 cm formaldehyde maser regions. We found no evidence of significant variability of the 6 cm masers in these regions with respect to previous observations, the only possible exception being the maser in G29.96−0.02. All six sources were also observed in the H 13 2 CO isotopologue transition of the 6 cm H 2 CO line; H

We present new Very Large Array 6 cm H 2 CO observations toward four extragalactic radio continuum sources (B0212+735, 3C 111, NRAO 150, BL Lac) to explore the structure of foreground Galactic clouds as revealed by absorption variability. This project adds a new epoch in the monitoring observations of the sources reported by Marscher and collaborators in the mid 1990's. Our new observations confirm the monotonic increase in H 2 CO absorption strength toward NRAO 150. We do not detect significant variability of our 2009 spectra with respect to the 1994 spectra of 3C111, B0212+735 and BL Lac; however we find significant variability of the 3C111 2009 spectrum with respect to archive observations conducted in 1991 and 1992. Our analysis supports that changes in absorption lines could be caused by chemical and/or geometrical gradients in the foreground clouds, and not necessarily by small scale (∼ 10 AU) high density molecular clumps within the clouds.