Extremely low mass white dwarfs are very rare objects likely formed in compact binary systems. We present MMT optical spectroscopy of 42 low-mass white dwarf candidates serendipitously discovered in a survey for hypervelocity B-type stars. One of these objects, SDSS J0917+46, has Teff=11,288+/-72 K and logg=5.48+/-0.03 with an estimated mass of 0

The amount of information available in spectropolarimetric data is estimated. To this end, the intrinsic dimensionality of the data is inferred with the aid of a recently derived estimator based on nearest neighbor considerations and obtained applying the principle of maximum likelihood. We show in detail that the estimator correctly captures the

Context: The geometry and physical conditions in solar coronal loops are complicated and still not understood well. Recent high-resolution observations obtained with TRACE indicate the existence of sub-resolution transverse structuring not accessible to direct observation. This ingredient has not yet been taken into account in the previous

The Ca II infrared (IR) triplet lines are a promising candidate for studying chromospheric magnetism and dynamics. To study how good of a diagnostic the lines are for chromospheric magnetism in the quiet Sun we have constructed a MHD simulation in the high plasma-β regime, analyzed quiet Sun spectropolarimetric data of the lines and used a non

Context: The very young σ Orionis cluster (~3 Ma) is a cornerstone in understanding the formation of stars and substellar objects down to planetary masses. However, its stellar population is far from being completely known. Aims: This study's purpose is to identify and characterise the most massive stars of σ Orionis to complement current and

We studied the brightness and spectral evolution of the young eruptive star V1647 Ori during its recent outburst in the period 2004 February-2006 September. We performed a photometric follow-up in the bands V, RC, IC, J, H, and Ks, as well as visible and near-IR spectroscopy. The main results derived from combining our data with those published by