In this paper 210 photographic positions of 13 asteroids obtained with the G.P.O. astrograph (D = 40 cm and F = 400 cm) at the European Southern Observatory (La Silla, Chile) in September 1987 are given. As for the observations made in La Silla, the now standard technique of the 3 exposures slightly spaced in declination (Debehogne 1985) was used. It replaced the old Uccle method based on only one exposure of one hour exposure time. This new technique, compared to the old method, has the advantage of a better measurement and an easier identification of the minor planet. The measurements of these plates were made with the Uccle Ascorecord coordinatograph and as for the reduction 5 reference stars and the dependence method were used. The reference stars were taken from the S.A.O. catalogue and an intermediate one made at Uccle. If necessary, stars, whose originally unknown positions were determined from measurements on other plates, were used. The use of only 5 stars as reference is justified by the focal length of the G.P.O. astrograph and by its high optical quality. Moreover, when there is a large number of plates to be reduced, this method becomes indispensable in order to contain the measurement time within acceptable limits. The typical error on reference stars is of Ol'4 in both coordinates. The results are presented in Table . The stars residuals and dependences are given in a table, which cannot be published in this paper because of lack of space. Readers interested in these data may contact the authors or the editorial office of the journal. Particularly, the residuals of Table were obtained using the ephemerides issued on Ephemerides of Minor Planets (Lavrov 1987).