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Recent observations show that the OH 1720 MHz maser is a powerful probe ofthe shocked region where a supernova remnant strikes a molecular cloud. Weperform a thorough study of the pumping of this maser and find tightconstraints on the physical conditions needed for its production. The presenceof the maser implies moderate temperatures (50 -- 125 K) and densities ($\sim10^5 cm^{-3}$), and OH column densities of order $10^{16} cm^{-2}$. We showthat these conditions can exist only if the shocks are of C-type. J-shocks failby such a wide margin that the presence of this maser could become the mostpowerful indicator of C-shocks. These conditions also mean that the 1720 MHzmaser will be inherently weak compared to the other ground state OH masers. Allthe model predictions are in good agreement with the observations.Comment: 16 pages, 5 Postscript figures (included), uses aaspp4.sty. To appear  in the Astrophysical Journa

OH 1720 Megahertz Masers in Supernova Remnants: C‐Shock Indicators