Abstract. Dessins d'enfants can be regarded as bipartite graphs embedded in compact orientable surfaces. According to Grothendieck and others, a dessin uniquely determines a complex structure on the surface, and even an algebraic structure (as a projective algebraic curve defined over a number field). The general problem of how to determine all properties of the curve from the combinatorics of the dessin is far from being solved. For regular dessins, which are those having an edge-transitive automorphism group, the situation is easier: currently available methods in combinatorial and computational group theory allow the determination of the fields of definition for all curves with regular dessins of genus 2 to 18.
The many facets of dessinsOne may introduce dessins d'enfants as hypermaps on compact oriented 2-manifolds. These objects, first studied in genus 0 by Cori in [9], have several equivalent algebraic or topological definitions. Topologically they are a generalisation of maps, whose underlying graphs are replaced with hypergraphs, in which hyperedges are allowed to be incident with any finite number of hypervertices and hyperfaces. One way to visualise this concept is to represent a hypermap D by its Walsh map W (D) , a connected bipartite graph B embedded in a compact oriented surface, dividing it into simply connected cells [31]. In the language of hypermaps, the white and black vertices represent the hypervertices and hyperedges of D, the edges represent incidences between them, and the cells represent the hyperfaces.There are several other ways to define dessins. There is a group theoretic description of maps and hypermaps by their (hyper)cartographic groups; see, for example, [18], [19] or the introduction of [20]. The latter are the monodromy groups of the functions about to be introduced, and give a link to function theory on Riemann surfaces and algebraic geometry on curves. Playing a key role are