Doppler measurements from Subaru and Keck have revealed radial velocity variations in the V ¼ 8:15, G0 IV star HD 149026 consistent with a Saturn-mass planet in a 2.8766 day orbit. Photometric observations at Fairborn Observatory have detected three complete transit events with depths of 0.003 mag at the predicted times of conjunction. HD 149026 is now the second-brightest star with a transiting extrasolar planet. The mass of the star, based on interpolation of stellar evolutionary models, is 1:3 AE 0:1 M ; together with the Doppler amplitude K 1 ¼ 43:3 m s À1 , we derive a planet mass M sin i ¼ 0:36M J and orbital radius 0.042 AU. HD 149026 is chromospherically inactive and metal-rich with spectroscopically derived ½ Fe/ H ¼ þ0:36, T eA ¼ 6147 K, log g ¼ 4:26, and v sin i ¼ 6:0 km s À1 . Based on T eff and the stellar luminosity of 2.72 L , we derive a stellar radius of 1.45 R . Modeling of the three photometric transits provides an orbital inclination of 85N3 AE 1N0 and (including the uncertainty in the stellar radius) a planet radius of (0:725 AE 0:05) R J . Models for this planet mass and radius suggest the presence of a $67 M È core composed of elements heavier than hydrogen and helium. This substantial planet core would be difficult to construct by gravitational instability.

We describe the University of Vienna twin automatic photoelectric telescope (APT) located at the new Fairborn Observatory at Washington Camp in southern Arizona. Not only are the telescopes fully automatic, but the observatory itself is automatic. A site-control computer monitors weather sensors and operates the roof while the telescope control computer operates the photometer and accepts input files from and provides nightly observations to the astronomer; no direct operating personnel are needed. Both telescopes have 0.75-m primary mirrors, a CCD finder camera, and an environmentally controlled single-channel photoelectric photometer with filter combinations optimized for use in the blue (Wolfgang) and in the red (Amadeus) wavelength regions. The telescopes are currently used for monitoring spotted stars, for asteroseismology of nonradially pulsating variables, and for monitoring asymptotic-giant-branch stars. Some first results are presented.

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We review the factors limiting the precision of differential stellar photometry. Errors due to variable atmospheric extinction can be reduced to below 0.001 mag at good sites by utilizing the speed of robotic telescopes. Existing photometric systems produce aliasing errors, which are several millimagnitudes in general but may be reduced to about a millimagnitude in special circumstances. Conventional differential photometry neglects several other important effects, which we discuss in detail. If all of these are properly handled, it appears possible to do differential photometry of variable stars with an overall precision of 0.001 mag with ground-based robotic telescopes.