With the 1 m Schmidt telescope of the Llano del Hato Observatory and the QUEST CCD camera, 380 deg 2 of the sky have been surveyed for RR Lyrae variables in a band 2 .3 wide in declination (centered at = À1) and covering right ascensions from 4C1 to 6C1 and from 8C0 to 17C0. The bright limit (due to CCD saturation) and the faint limit are V $13.5 and $19.7, respectively, which correspond to $4 and $60 kpc from the Sun. We present a catalog of the positions, amplitudes, mean magnitudes, periods, and light curves of the 498 RR Lyrae variables that have been identified in this region of the sky. The majority of these stars (86%) are new discoveries. The completeness of the survey has been estimated from simulations that model the periods and light curves of real RR Lyrae variables and take into account the pattern of epochs of observation. While the completeness of the survey varies with apparent magnitude and with position, almost everywhere in the surveyed region it is quite high (>80%) for the type ab RR Lyrae variables and between 30% and 90% for the lowamplitude type c variables.

Pluto's tenuous nitrogen atmosphere was first detected by the imprint left on the light curve of a star that was occulted by the planet in 1985 (ref. 1), and studied more extensively during a second occultation event in 1988 (refs 2-6). These events are, however, quite rare and Pluto's atmosphere remains poorly understood, as in particular the planet has not yet been visited by a spacecraft. Here we report data from the first occultations by Pluto since 1988. We find that, during the intervening 14 years, there seems to have been a doubling of the atmospheric pressure, a probable seasonal effect on Pluto.

[1] We report the observation of two stellar occultations by Titan on 14 November 2003, using stations in the Indian Ocean, southern Africa, Spain, and northern and southern Americas. These occultations probed altitudes between $550 and 250 km ($1 to 250 mbar) in Titan's upper stratosphere. The light curves reveal a sharp inversion layer near 515 ± 6 km altitude (1.5 mbar pressure level), where the temperature increases by 15 K in only 6 km. This layer is close to an inversion layer observed fourteen months later by the Huygens HASI instrument during the entry of the probe in Titan's atmosphere on 14 January 2005 [Fulchignoni et al., 2005]. Central flashes observed during the first occultation provide constraints on the zonal wind regime at 250 km, with a strong northern jet ($200 m s À1 ) around the latitude 55°N, wind velocities of $150 m s À1 near the equator, and progressively weaker winds as more southern latitudes are probed. The haze distribution around Titan's limb at 250 km altitude is close to that predicted by the Global Circulation Model of Rannou et al. (2004) in the southern hemisphere, but a clearing north of 40°N is necessary to explain our data. This contrasts with Rannou et al.'s (2004) model, which predicts a very thick polar hood over Titan's northern polar regions. Simultaneous observations of the flashes at various wavelengths provide a dependence of t / l Àq , with q = 1.8 ± 0.5 between 0.51 and 2.2 mm for the tangential optical depth of the hazes at 250 km altitude.

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We have designed, constructed and put into operation a large area CCD camera that covers a large fraction of the image plane of the 1 meter Schmidt telescope at Llano del Hato in Venezuela. The camera consists of 16 CCD devices arranged in a 4 × 4 mosaic covering 2.3 • × 3.5 • of sky. The CCDs are 2048 × 2048 LORAL devices with 15 µm pixels. The camera is optimized for drift scan photometry and objective prism spectroscopy. The design considerations, construction features and performance parameters are described in the following article.Subject headings: instrumentation: detectors, surveys, cosmology IntroductionSchmidt telescopes are the instrument of choice for surveys of large areas of the sky because of their large field, typically of the order of 4 • ×4 • . However, these telescopes have large, curved image planes and are difficult to instrument with silicon detectors. Until now, these telescopes have been used with photographic plates and no Schmidt telescope had its image plane fully instrumented with silicon CCD detectors. The Near-Earth Asteroid Tracking (NEAT) project has recently instrumented the Palomar 48" Oschin Schmidt telescope with three 4080 × 4080 CCDs (Pravdo 2002). This camera covers a total of ∼ 3.75 square degrees of the ∼ 36 square degree field of view. A project is underway to fully instrument this telescope.We, the QUEST collaboration, 9 have designed, constructed and put into operation, a large area CCD camera that covers a large fraction of the image plane of the 1 meter Schmidt telescope at the Venezuelan National Astronomical Observatory located at Llano del Hato and operated by CIDA. 10

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The 1973È1999 V -band light curve from AAVSO visual estimates and RoboScope CCD exposures is examined to study the systematic properties of normal outbursts and superoutbursts in this prototype of the SU UMaÈtype dwarf novae. A number of the outburstsÏ correlations previously reported in VW Hyi and in SU UMa are not present in these data. Several lines of evidence suggest that, during this 26 year interval, variations in the mass-transfer rate served to obscure some of the expected correlations. The M 0 variation apparently caused a nearly complete cessation of outbursting in 1980È1983. We do Ðnd a correlation of the time since the last superoutburst with the mean brightness in the interval since the last superoutburst, supporting the notion that superoutbursts are a disk-initiated phenomenon that occurs when the gas remaining in the disk after each normal outburst has accumulated to some critical value.

We describe the discovery circumstances and photometric properties of 2000 EB173, now one of the brightest trans-Neptunian objects (TNOs) with opposition magnitude m R = 18.9 and also one of the largest Plutinos, found with the drift-scanning camera of the QUEST Collaboration, attached to the 1-m Schmidt telescope of the National Observatory of Venezuela. We measure B − V = 0.99 ± 0.14 and V − R = 0.57 ± 0.05, a red color observed for many fainter TNOs. At our magnitude limit m R = 20.1 ± 0.20, our single detection reveals a sky density of 0.015 (+0.034, -0.012) TNOs per deg 2 (the error bars are 68% confidence limits), consistent with fainter surveys showing a cumulative number proportional to 10 0.5m R . Assuming an inclination distribution of TNOs with FWHM exceeding 30 deg, it is likely that one hundred to several hundred objects brighter than m R = 20.1 remain to be discovered.