Red giants in open clusters. V. NGC 2099

474 radial-velocity observations and new UBV photoelectric data for 55 and 20 red giants respectively, in the field of the intermediate-age open cluster NGC 2099 are presented and analysed for membership and duplicity. The membership of 35 red giants has been confirmed, 16 spectroscopic binaries have been discovered and 11 orbits have been determined. The cluster binary-frequency lower limit is slightly above the average (9/35=26%). The radial distribution of the red giants shows a sharp limit between members and non-members. No mass segregation is observed between the binary and single red giants. The latest isochrones from the Geneva and Padova groups reproduce very well the observed morphology in the colour-magnitude diagram for log t=8.65 and z=0.02, with (m-M)=11.50 and E(B-V)=0.29.