Early-type stars observed in the ESO UVES Paranal observatory project - IV. studies of CN, CH<sup>+ and CH in the interstellar medium

High spectral resolution (~80 000) and signal-to-noise observations from the Ultraviolet and Visual Echelle Spectrograph Paranal Observatory Project (UVES-POP) are used to study the interstellarmolecular lines CN (3874 Å), CH<sup>+ (3957, 4232 Å) and CH (3886, 4300 Å) towards 74 O- and B-type stellar sightlines. Additionally, archive data are presented for 140 ELODIE early-type stellar sightlines at R = 42 000, plus 25 FEROS at R = 48 000 and 3 UVES at R > 50 000, mainly in the CH<sup>+ (4232 Å) and CH (3886, 4300 Å) transitions. Detection rates are ~45 per cent for CN and ~67 per cent for the other lines in the POP sample, and ~10-15 per cent for CH<sup>+ and CH lines in the additional sample. CH and CH<sup>+ are well correlated between log[N(CH) cm^-2]~12-14, implying that these clouds are CH<sup>+-like CH and not CN-like CH. CH is also very well correlated with Na I D in the range log[N(Na I cm^-2]) ~12.2-14.2. A few sightlines show tentative velocity shifts of ~2 km s^-1 between CH and CH<sup>+, which appear to be caused by differences in component strength in blends, and hence do not provide firm evidence for shocks. Finally, we describe a search for ¹³CH<sup>+ in a sightline towards HD 76341. No ¹³CH<sup>+ is detected, placing a limit on the ¹³CH<sup>+ to ¹²CH<sup>+ ratio of ~0.01. If a formal fit is attempted, the equivalent width ratio in the two isotopes is a factor ~90 but with large errors.