The atmospheric extinction at the complejo astronómico el leoncito and the Bosque Alegre station

The first-and second-order extinction coefficients in the UBVRI and DDO systems are derived for the first time at CASLEO. They define an extinction curve which is discussed in the framework of atmospheric absorption theory. From this curve the first-order extinction coefficients for the Washington and Strömgren systems are inferred. From a photometric point of view, CASLEO occupies a preferred place among the observatories of the world. A comparison of the UBV extinction coefficients obtained at Bosque Alegre with previous determinations supports the conclusion that the sky transparency at this station has deteriorated in the ultraviolet in the last years.