The authors present preliminary results of an infrared monitoring program of selected blazars, conducted at the Carlos Sanches Telescope at Izana Observatory, Teneriffe. Eight blazars have been observed during three observing runs since August 1990. All observed blazars have shown variability.

The paper presents simultaneous monitoring of the BL Lacertae object Markarian 501 obtained in the visible and near infrared using a double photometer system on the 1.5 m Carlos Sanchez Telescope situated in El Observatorio del Teide (Tenerife) and CCD photometry in U and B taken with the 1 m Jacobus Kapteyn Telescope of El Observatorio del Roque

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Two alternative methods for evaluating the probability densities of structures defined by d degrees of freedom in random fields are presented. For Gaussian random fields, both differentiable and nondifferentiable, the application of these methods is considered in detail. The relationship between structures, maxima, and connected regions above a

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Results of high-resolution optical spectroscopy of V404 Cyg (the optical counterpart of the X-ray transient GS2023 + 338), conducted 13 months after the May 1989 outburst are presented. The double-peaked H-alpha line exhibits a strong S-wave modulation from which a periodicity of 5.7 hr is deduced. The level of the continuum is also modulated on