A stringy black string and a stringy black hole in four dimensions

We uplift the static three dimensional black hole solution found by Banados, Teitelboim and Zanelli (BTZ) into four dimensional space time. In this way we obtain a black string solution with a relativistic string source, as well as a new black hole solution which is also generated by a relativistic stringy'' source. It is shown that when passing continuously from the region of the one dimensional parameter spaces which characterize these solutions, containing naked singular strings or naked singular points’‘, and into the region containing black strings or black holes, the metrics blow up'' at a critical point in both parameter spaces. We show that a similar separation’’ mechanism can be introduced in three dimensions. In this way we also obtain a generalization of the BTZ solution. The separation'' mechanism, and its immediate consequences, offers an hitherto missing technical argument needed in order to exclude the naked singularities (the mass gap’‘) from the space of ``physically permissible’’ solutions in three dimensional Einstein theory coupled to a negative cosmological constant. PACS nos.: 04.40.Nr, 04.20.Cv