Abstract. We performed long-term photometry of the young solar analog EK Draconis (HD 129333) using 1030 blue-sensitive Sky-Patrol plates taken at Sonneberg Observatory from 1958 onwards. A secular dimming of 0.0057 ± 0.0008 mag/y beginning around 1975 was found. Hence, the recent trend first observed with Automatic Photoelectric Telescopes (APTs) is confirmed and traced back to the mid-seventies.

Abstract.We look for a correlation between Lithium abundance and rotation period in solar-type stars (G0. . . K0). The result is not unambiguous. Stars in young open clusters (IC 2391, IC 2602, IC 4665, α Per) do not show any correlation, in the Pleiades cluster a correlation is present: the faster the rotation the less the Lithium depletion. Moreover, there is statistical evidence, that at least in the Pleiades and in the α Persei cluster the abundance spread at fixed color in cool stars could be due to rotation. The use of the inverse Rossby number instead of the rotation rate does not improve on the Li-rotation correlation.

A B S T R A C TThe phenomenon of negative viscosity-alpha in convectively unstable Keplerian accretion discs is discussed. The convection is considered as a random flow with an axisymmetric mesoscale pattern. Its correlation tensor is computed with a time-averaging procedure using Kley's 2D hydrocode. There is a distinct anisotropy between the turbulence intensities in the radial and azimuthal directions, i.e. the radial velocity rms dominates the azimuthal one. As a consequence, an extra term in the expression for the turbulent transport of angular momentum appears which does not vanish for rigid rotation ('L-effect'). It is negative ('inwards transport') and even seems to dominate the positive contribution of the eddy viscosity representing outwards transport of angular momentum. For a turbulence model close to that of the mixing-length theory, the rotational influence on the anisotropy of the turbulence intensities, ku 0 2 R l 2 ku 0 2 f l, and the covariance ku 0 R u 0 f l -representing the angular momentum transport -is computed and compared with the accretion disc simulations. Indeed, the negative angular momentum transport can be explained with the observed dominance of the radial turbulence intensity. If, on the other hand, in turbulence fields the azimuthal intensity would dominate or the turbulence is even isotropic, then we always find a positive transport of the angular momentum.

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The idea that the TitiusrBode law reflects an unstable mode of a self-gravitational instability in very thin Keplerian disks makes a careful discussion of the Poisson equation especially necessary. Due to the planetary distances in the solar system (6r/r 2 0.5) the well-known short-wave approximation is not appropriate for definite assertions. We will here use a simple series expansion of the relation between the radial and vertical wave numbers of the disturbances which is additionally valid for medium-scale and non-zonal modes. The numerical solution of the dispersion relation reveals an extra unstable branch for wave-lengths of rings and spirals two orders of magnitudes larger than those already known. Though we are not yet able to consider modes long enough for application to the planetary system, we feel the existence of the medium-wave instability (6r/r = 0.1) to be a serious challenge for a better, i.e. non-local theory.Die Idee, daD die Titius-Bodesche Reihe die instabile Mode einer gravitativen Instabilitat in einer dunnen Gasscheibe darstellt, erfordert eine sorgfaltige Behandlung der Poisson-Gleichung. Die haufig benutzte Kurzwellen-Approximation ist wegen der Abstandsverhaltnisse im Planetensystem ( F r / r 2: 0.5) fur giiltige Aussagen ungeeignet. Die von uns verwendete Reihenentwicklung in der Beziehung zwischen radialer und vertikaler Wellenzahl erlaubt auch die Behandlung ma0ig-langer sowie nicht-axialsymmetrisher Storungen. Die vorwiegend numerische Diskussion der Dispersionsrelation IaBt das Vorhandensein einer zudtzlichen instabilen Mode erkennen, deren Wellenlange die der bisher allein bekannten um etwa das Hundertfache ubersteigt. Obwohl selbst damit die geometrischen Verhaltnisse des solaren Planetensystems noch nicht wiedergegeben werden, halten wir die Existenz instabiler Mittelwellen (Fr/r 2: 0.1) doch fur eine ernste Herausforderung zur Konstruktion besserer, also nicht-lokaler Ansitze.