The Microvariablity and Oscillations of Stars (MOST ) mission is a low-cost microsatellite designed to detect low-degree acoustic oscillations (periods of minutes) with micromagnitude precision in solartype stars and metal-poor subdwarfs. There are also plans to detect light reflected from giant, short-period, extrasolar planets and the oscillations of roAp stars and the turbulent variability in the dense winds of Wolf-Rayet stars. This paper describes the experiment and how we met the challenge of ultraprecise photometry despite severe constraints on the mass, volume, and power available for the instrument. A side-viewing, 150 mm aperture Rumak-Maksutov telescope feeds two frame-transfer CCDs, one for tracking and the other for science. There is a single 300 nm wide filter centered at 525 nm. Microlenses project Fabry images of the brighter ( ) target V ≤ 10 stars onto the science CCD. Fainter target stars will be focused directly elsewhere on the CCD. MOST was launched on 2003 June 30 into a low-Earth, Sun-synchronous, polar orbit allowing stars between Ϫ19Њ and ϩ36Њ declination to be viewed continuously for up to 60 days. Attitude is controlled by reaction wheels and magnetotorquers. A solar safety shutter over the telescope diagonal is the only other moving part. Accumulated photometry will be used to calibrate response across the target field stop, and data will be compressed and downloaded to three dedicated ground stations.

We have detected transits of the innermost planet "e" orbiting 55 Cnc (V = 6.0), based on two weeks of nearly continuous photometric monitoring with the MOST space telescope. The transits occur with the period (0.74 d) and phase that had been predicted by Dawson & Fabrycky, and with the expected duration and depth for the crossing of a Sun-like star by a hot super-Earth. Assuming the star's mass and radius to be 0.963 +0.051 −0.029 M ⊙ and 0.943 ± 0.010 R ⊙ , the planet's mass, radius, and mean density are 8.63 ± 0.35 M ⊕ , 2.00 ± 0.14 R ⊕ , and 5.9 +1.5 −1.1 g cm −3 . The mean density is comparable to that of Earth, despite the greater mass and consequently greater compression of the interior of 55 Cnc e. This suggests a rock-iron composition supplemented by a significant mass of water, gas, or other light elements. Outside of transits, we detected a sinusoidal signal resembling the expected signal due to the changing illuminated phase of the planet, but with a full range (168 ± 70 ppm) too large to be reflected light or thermal emission. This signal has no straightforward interpretation and should be checked with further observations. The host star of 55 Cnc e is brighter than that of any other known transiting planet, which will facilitate future investigations.

We have attempted to establish an observational evidence for presence of distant companions which may have acquired and/or absorbed the angular momentum during evolution of multiple systems thus facilitating or enabling formation of contact binaries. In this preliminary investigation we use several techniques (some of them distance-independent) and mostly disregard detection biases of individual techniques in an attempt to establish a lower limit to the frequency of triple systems. While the whole sample of 151 contact binary stars brighter than V max = 10 mag. gives a firm lower limit of 42% ± 5%, the corresponding number for the much better observed Northern-sky sub-sample is 59% ±8%. These estimates indicate that most contact binary stars exist in multiple systems.

Hipparcos parallax data for 40 contact binary stars of the W UMa-type (with ǫM V < 0.5) are used to derive a new, (B − V )-based absolute-magnitude calibration of the form M V = M V (log P, B − V ). The calibration covers the ranges 0.26 < (B − V ) 0 < 1.14, 0.24 < P < 1.15 day, and 1.4 < M V < 6.1; it is based on a solution weighted by relative errors in the parallaxes (2.7% to 24%). Previous calibrations have not been based on such a wide period and color space, and while they have been able to predict M V with sufficient accuracy for systems closely following the well-known period-color relation, the new calibration should be able to give also good predictions for more exotic "outlying" contact binary systems. The main limitations of this calibration are the inadequate quality of the ground-based photometric data, and the restriction to the (B − V ) index, which is more sensitive to metallicity effects than the (V − I) index; metallicities are, however, basically unknown for the local W UMa-type systems.

The Microvariability and Oscillations of Stars (MOST) satellite observed the B supergiant HD 163899 (B2 Ib/II) for 37 days as a guide star and detected 48 frequencies P 2.8 cycles day À1 with amplitudes of a few millimagnitudes (mmag) and less. The frequency range embraces gand p-mode pulsations. It was generally thought that no g-modes are excited in less luminous B supergiants because strong radiative damping is expected in the core. Our theoretical models, however, show that such g-modes are excited in massive postYmain-sequence stars, in accordance with these observations. The nonradial pulsations excited in models between 20 M at log T eA % 4:41 and 15 M at log T eA % 4:36 are roughly consistent with the observed frequency range. Excitation by the Fe bump in opacity is possible because g-modes can be partially reflected at a convective zone associated with the hydrogen-burning shell, which significantly reduces radiative damping in the core. The MOST light curve of HD 163899 shows that such a reflection of g-modes actually occurs and reveals the existence of a previously unrecognized type of variable, slowly pulsating B supergiants (SPBsg) distinct from Cyg variables. Such g-modes have great potential for asteroseismology.