Context. Asteroseismology has great potential for the study of metal-poor stars due to its sensitivity to determine stellar ages. Solid detections of oscillation frequencies in stars with well constrained fundamental parameters, combined with a known rotation period, should significantly advance our understanding of stellar structure and evolution in context with metallicity effects. Aims. Our goal was to detect p-mode oscillations in the metal-poor sub-dwarf 85 Peg A and to search for variability on longer timescales. Methods. We have obtained continuous high-precision optical photometry of the binary system 85 Pegasi with the MOST (Microvariability & Oscillations of STars) space telescope in two seasons (2005 & 2007). The light curves were analyzed using traditional Fourier techniques. Furthermore, we redetermined $v\sin i$ for 85 Peg A using high resolution spectra obtained through the ESO archive, and used photometric spot modeling to interpret long periodic variations. Results. Our frequency analysis yields no convincing evidence for p-modes significantly above a noise level of 4 ppm. Using simulated p-mode patterns we provide upper rms amplitude limits for 85 Peg A. After removal of instrumental trends the light curve shows evidence for variability with a period of about 11 d and this periodicity is also seen in the follow up run in 2007; however, as different methods to remove instrumental trends in the 2005 run yield vastly different results, the exact shape and periodicity of the 2005 variability remain uncertain. Our re-determined $v\sin i$ value for 85 Peg A is comparable to previous studies and we provide realistic uncertainties for this parameter. Using these values in combination with simple photometric spot models we are able to reconstruct the observed variations. Conclusions. The null-detection of p-modes in 85 Peg A is consistent with theoretical values for pulsation amplitudes in this star. The detected long-periodic variation in the 85 Peg system must await confirmation by further observations with similar or better precision and long-term stability. If the 11 d periodicity is real, rotational modulation of surface features on one of the components is the most likely explanation.
