PhotoelectricUBV and DDO photometry of NGC 5138

Results ofUBV photoelectric photometry in NGC 5138 are presented for 50 stars brighter than 14.0 mag. In addition, four probable red giants were also observed in the DDO system. Sixteen stars previously considered members by Lindoff (1972), were found not to be physically connected with the cluster. NGC 5138 is located 1.80 kpc from the Sun and the visual interstellar absorption determined from the reddenedB stars amounts toA _v =0.75 mag. Three of the four red stars observed in the DDO system were found to be cluster members. The mean cyanogen anomaly is CN=0.043±0.018 (m.e.), which implies that NGC 5138 is richer in CN than the field K giants in the solar neighbourhood, but poorer than the Hyades giants. The cluster age is estimated to be 1.5×10⁸ yr.Member of the Carrera del Investigador Cientifico del Consejo Nacional de Investigaciones Científicas y Técnicas de la República Argentina, and visiting astronomer, Cerro Tololo Inter-American Observatory, operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation.