CO, HCO+ AND HCN ABSORPTION IN THE GRAVITATIONAL LENS CANDIDATE B0218+357 AT Z=0.685

Absorption lines of CO(1-2), CO(2-3), HCO+(1-2) and HCN(1-2) have been detected at a redshift of z(a) = 0.68466 towards the background BL La c object B0218+357. The CO and HCN lines are strongly saturated, while the HCO+ line is mildly saturated. Lower limits to the column densiti es are (7.0 +/- 0.3) x 10(16) cm(-2) for CO, (7.4 +/- 0.2) x 10(13) cm (-2) for HCO+ and (1.4 +/- 0.1) x 10(14) cm(-2) for HCN. The filling f actor for the background source is larger than 0.7-0.8. The absorption line profile is asymmetric with a width of similar to 30 km s(-1). B0 218+357 is a gravitational lens candidate with two compact continuum s ources separated by 335 mas (1.8 kpc). The small linewidth of the mole cular absorption puts constraints on the inclination and position angl e of the lensing galaxy. We also report a search for QSOs molecular ab sorption lines in 12 other redshifted systems; none were detected.