Pulsation, binarity and close frequency spacing in delta Scuti stars: BQ and BW Cancri - Results of the STEPHI VI campaign in Praesepe cluster

New pulsation modes in two delta Scuti stars of Praesepe cluster, BQ and BW Cnc, have been detected during the STEPHI VI campaign in 1995. In particul ar, 3 frequencies for BQ Cnc and 9 frequencies for BW Cnc have been found a bove a 99% confidence level. The possibility of the presence of a g-mode pr esent in BQ Cnc is discussed, considering its binarity. The effect of mutua l interference between very close detected frequencies in BW Cnc during the observations, is also considered. This last effect reveals the necessity o f long-period observing runs, in order to avoid its influence in the final number of detected modes. In such situation, studies of secular amplitude c hanges can be strongly affected.