The old open clusters Berkeley 36, Berkeley 73 and Biurakan 13 (Berkeley 34) ^*,**

¹ Università di Padova, Dipartimento di Astronomia, Vicolo dell'Osservatorio 2, 35122 Padova, Italy e-mail: ortolani@pd.astro.it
² Universidade Federal do Rio Grande do Sul, Dept. de Astronomia, CP 15051, Porto Alegre 91500-970, Brazil e-mail: bica@if.ufrgs.br
³ Universidade de São Paulo, Rua do Matão 1226, 05508-900 São Paulo, Brazil e-mail: barbuy@astro.iag.usp.br
⁴ Universidad Catolica de Chile, Department of Astronomy & Astrophysics, Casilla 305, Santiago 22, Chile e-mail: mzoccali@astro.puc.cl

Received: 12 June 2004
Accepted: 10 August 2004

Abstract

BV photometry of the faint open clusters Berkeley 36, Berkeley 73 and Biurakan 13 are studied. For these clusters no colour–magnitude analyses were previously available. The colour–magnitude Diagrams indicate that they are all old open clusters. The derived ages are ~4 Gyr for Berkeley 36, and ~2.3 Gyr for both Berkeley 73 and Biurakan 13. The clusters are not very reddened, with  = 0.25, 0.10 and 0.30 respectively for Berkeley 36, Berkeley 73 and Biurakan 13. Berkeley 36 and Berkeley 73 are located at a distance from the Galactic center of ~10 kpc, while Biurakan 13 is much farther, at 15 kpc. A peak in the age distribution appears at 5 Gyr suggesting a distinct star forming event in the disk.