  Aims: We present accurate photometric time series of two Be stars: NW Ser and
V1446 Aql. Both stars were observed at the Observatorio de Sierra Nevada
(Granada) in July 2003 with an automatic four-channel Str\"omgren photometer.
We also present a preliminary theoretical study showing that the periodic
variations exhibited by these stars can be due to pulsation.
  Methods: An exhaustive Fourier analysis together with a least-square fitting
has been carried out on the time series for all four Str\"omgren bands. Several
independent frequencies and non-periodic trends explain most of the variance. A
theoretical non-adiabatic code applied to stellar models for these stars shows
that g-modes are unstable.
  Results: Both stars show rapid variations in amplitude, probably due to a
beating phenomenon. Four significant frequencies have been detected for each
star. Comparison of the observed amplitude ratios for each pulsational
frequency with those calculated from theoretical pulsation codes allows us to
estimate the pulsation modes associated with the different detected
frequencies. NW Ser seems also to show unstable p-modes and thus could be one
of the newly discovered $\beta$ Cephei and SPB hybrid stars. Further
spectroscopic observations are planned to study the stability of the detected
frequencies.
