delta Scuti stars in Praesepe - I. The STACC 1998 campaign - the photometry

We present the results of the multisite differential CCD photometry for the two delta Scuti stars, BN and BV Cnc, in the open cluster Praesepe. The ma in objective was to identify the character of the pulsation modes in BN Cnc deriving their accurate periods, amplitudes and phases. These parameters a re essential for the mode identification which uses combined photometric an d spectroscopic data and is presented in the second article. For BN Cnc, si x pulsation modes with amplitudes above the detection limit (similar to0.5 mmag) were detected. Using the same CCD frames it was possible to verify th e presence of the four pulsation modes in BV Cnc, the faintest of delta Scu ti stars in Praesepe. It is shown that in this very low-amplitude pulsator, substantial amplitude variations are seen between 1997 and 1998.