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E 11 t d e c k u n g e i n e s P 1 a 11 e t e n.Schreiben des Herrn Prof. C. H. F. Peters an den Herausgeber. E i n e s Lishcr noch nicht bekannten Planetcn rvurde icli zuerst gesterri Blorgen kurz \or Tagesanbruch ansichtig, erhielt bei schoo starker Dininierung die folgeride position : und 1867 Juli 7, 151)26m32s5 nrittl. Zeit.

Back-arc spreading centres and related volcanic structures are known for their intense hydrothermal activity. The axial volcanic edifice of Maka at the North Eastern Lau Spreading Centre is such an example, where fluids of distinct composition are emitted at the Maka hydrothermal field (HF) and at Maka South in 1,525–1,543 m water depth. At Maka HF black smoker-type fluids are actively discharged at temperatures of 329°C and are characterized by low pH values (2.79–3.03) and a depletion in Mg (5.5 mmol/kg) and SO4 (0.5 mmol/L) relative to seawater. High metal (e.g., Fe up to ∼6 mmol/kg) and rare Earth element (REE) contents in the fluids, are indicative for a rock-buffered hydrothermal system at low water/rock ratios (2–3). At Maka South, venting of white smoke with temperatures up to 301°C occurs at chimneys and flanges. Measured pH values range from 4.53 to 5.42 and Mg (31.0 mmol/kg), SO4 (8.2 mmol/L), Cl (309 mmol/kg), Br (0.50 mmol/kg) and Na (230 mmol/kg) are depleted compared to seawater, whereas metals like Li and Mn are typically enriched together with H2S. We propose a three-component mixing model with respect to the fluid composition at Maka South including seawater, a boiling-induced low-Cl vapour and a black smoker-type fluid similar to that of Maka HF, which is also preserved by the trace element signature of hydrothermal pyrite. At Maka South, high As/Co (>10–100) and Sb/Pb (>0.1) in pyrite are suggested to be related to a boiling-induced element fractionation between vapour (As, Sb) and liquid (Co, Pb). By contrast, lower As/Co (<100) and a tendency to higher Co/Ni values in pyrite from Maka HF likely reflect the black smoker-type fluid. The Se/Ge ratio in pyrite provides evidence for fluid-seawater mixing, where lower values (<10) are the result of a seawater contribution at the seafloor or during fluid upflow. Sulphur and Pb isotopes in hydrothermal sulphides indicate a common metal (loid) source at the two vent sites by host rock leaching in the reaction zone, as also reflected by the REE patterns in the vent fluids.

An in-tournament is a loopless digraph without multiple arcs and cycles of length 2 such that the negative neighborhood of every vertex induces a tournament. This paper tackles the problem of vertex k-pancyclicity in strong in-tournaments of order n, i.e., every vertex belongs to a cycle of length l for every k 6 l 6 n. In 2001, Tewes and Volkmann (J. Graph Theory 36 (2001) 84) gave sharp lower bounds for the minimum degree such that a strong in-tournament is vertex k-pancyclic for k 6 5 and k ¿ n − 3. In accordance with a family of examples (see J. Graph Theory 36 (2001) 84) and their results, they conjectured that every strong in-tournament of order n with minimum degree greater than 9(n−k−1) 5+6k+(−1) k 2 −k+2 + 1 is vertex k-pancylic. The main result of this paper is the conÿrmation of this conjecture for the case k = 6 (except for the values n = 14; 15; 16). Terminology and introductionThroughout this paper we will consider ÿnite digraphs without loops and multiple arcs. As usual for a digraph D its vertex set is denoted by V (D) and the arc set by E(D). For two distinct vertices u; v ∈ V (D), the notation u → v is used to indicate that there is an arc from u to v. We also write uv ∈ E(D) and say that u and v are adjacent, where u is called a negative neighbor of v and v is called a positive neighbor of u. Moreover, let D1 and D2 be two disjoint subdigraphs of D. If there is an arc from every vertex in D1 to every vertex in D2, then we say that D1 dominates D2, indicated by D1 → D2. In the case D1 = {z} (D2 = {ẑ}), we also use the short form z → D2 (D1 →ẑ).For an arbitrary vertex z ∈ V (D), we deÿne the positive neighborhood of z, N + (z) = N + (z; D), as the set of all positive neighbors of z in D. Analogously, N − (z) = N − (z; D) consists of all negative neighbors of z in D and is referred to as the negative neighborhood of z. Then we deÿne the indegree of z by d − (z) = d − (z; D) = |N − (z)| and accordingly the outdegree of z by d + (z) = d + (z; D) = |N + (z)|. More general, N + (X ) = N + (X; D) = z∈X N + (z) and N − (X ) = N − (X; D) = z∈X N − (z) for a subset X of V (D). For a subdigraph S of D, the positive and negative neighborhood of z with respect to S, N + (z; S) and N − (z; S), are given by N + (z) ∩ V (S) and N − (z) ∩ V (S), respectively. Analogously, we deÿne d + (z; S), d − (z; S), N + (X; S) and N − (X; S). Furthermore, d + (X; S) = |N + (X; S)| and d − (X; S) = |N − (X; S)|. Moreover, D[X ] denotes the subdigraph that is induced by the subset X of V (D). The minimum degree (D) of D is deÿned by (D) = min{ − (D); + (D)}, where − (D) = min z∈V (D) d − (z) and + (D) = min z∈V (D) d + (z).Whenever we refer to cycles and paths, we mean by this oriented cycles and oriented paths. A cycle C of length k is also called a k-cycle. We say that a digraph D is strongly connected or just strong, if for every pair of distinct vertices (u; v), u; v ∈ V (D), there is a path from u to v. A digraph D is connected, if the underlying graph is connected. We only consider connected digraphs in this paper.