Context. Radial velocity monitoring of very cool dwarfs such as late M- and hot L-dwarfs has become a promising tool in the search for rocky planets as well as follow-up planetary candidates around dwarfs detected by transit surveys. These stars are faint at optical wavelengths, as their spectral flux distribution peaks at near-infrared (NIR)

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Using archival Hubble Space Telescope (HST) imaging data, we report the multiband photometric properties of 13 ultraluminous X-ray sources (ULXs) that have a unique compact optical counterpart. Both magnitude and color variation are detected at timescales of days to years. The optical color, variability, and X-ray to optical flux ratio indicate

Type Ia supernovae are key tools for measuring distances on a cosmic scale. They are generally thought to be the thermonuclear explosion of an accreting white dwarf in a close binary system. The nature of the mass donor is still uncertain. In the single-degenerate model it is a main-sequence star or an evolved star, whereas in the double-degenerate

As part of an ongoing Chandra program for precise localization of X-ray transients in low-mass X-ray binaries, we obtained a short Chandra/ACIS-I observation of the X-ray transient Swift J174535.5-285921 during its 2011 July discovery outburst (ATEL #3472, #3476, #3481, #3508). The exposure time was 966 s, using a (1/8)-subarray of the I3 chip with

As part of an ongoing Chandra program for precise localization of transient low-mass X-ray binaries, we obtained a short Chandra observation of the 401 Hz transient accreting millisecond X-ray pulsar IGR J17498-2921 during its 2011 August discovery outburst (ATEL #3551, #3555, #3556). Our observation was made on 2011 August 22, 00:40 TT with an