With the object of obtain the extinction coefficients for the Estación Astronómica "Dr. Carlos Ulrrico Cesco" stars of the Standard Areas of Landolt (1973, 1983) have been observed during the period March 1991 - May 1992. An increasing in the primary extinction has been detected during the second semester of 1991 due mainly to the presence of particles in the atmosphere, while the secondary coefficients have been constant during this period. The mean values obtained for the first order extinction are: (U-B)= 0.22 ± 0.04, (B-V)= 0.15 ± 0.02 and V = 0.12 ± 0.03, and for the second order extinction are: (U-B)= 0.000 ± 0.010, (B-V)= -0.032 ± 0.006 and V=0.000 ± 0.009.Asociación Argentina de Astronomí
