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Context: Asteroseismology has great potential for the study of metal-poorstars due to its sensitivity to determine stellar ages. Aims: Our goal was todetect p-mode oscillations in the metal-poor sub-dwarf 85 Peg A and to searchfor other variability on longer timescales. Methods: We have obtainedcontinuous high-precision photometry of the binary system 85 Pegasi with theMOST space telescope in two seasons (2005 & 2007). Furthermore, we redeterminedvsini for 85 Peg A using high resolution spectra obtained through the ESOarchive, and used photometric spot modeling to interpret long periodicvariations. Results: Our frequency analysis yields no convincing evidence forp-modes significantly above a noise level of 4 ppm. Using simulated p-modepatterns we provide upper RMS amplitude limits for 85 Peg A. The light curveshows evidence for variability with a period of about 11 d and this periodicityis also seen in the follow up run in 2007; however, as different methods toremove instrumental trends in the 2005 run yield vastly different results, theexact shape and periodicity of the 2005 variability remain uncertain. Ourre-determined vsini value for 85 Peg A is comparable to previous studies and weprovide realistic uncertainties for this parameter. Using these values incombination with simple photometric spot models we are able to reconstruct theobserved variations. Conclusions: The null-detection of p-modes in 85 Peg A isconsistent with theoretical values for pulsation amplitudes in this star. Thedetected long-periodic variation must await confirmation by furtherobservations with similar or better precision and long-term stability. If the11 d periodicity is real, rotational modulation of surface features on one ofthe components is the most likely explanation.

A search for p-modes and other variability in the binary system 85 Pegasi using MOST photometry