We explore the shadow of certain class of rotating traversable wormholes within classical general relativity. The images depend on the angular momentum of the wormhole, and the inclination angle of the observer. We compare the results with the case of the Kerr black hole. For small angular momenta the shadows for the two solutions are nearly identical, however with the increasing of the angular momentum they start to deviate considerably.

We revisit the shadow of rotating traversable wormholes discussing the role of the wormhole throat in the shadow formation. For certain classes of wormholes the throat serves as a potential barrier for light rays with particular impact parameters, thus modifying the shadow shape. We consider a couple of wormhole solutions and examine the structure of their shadow images, and the intrinsic mechanisms for their formation. Some of the shadows possess cuspy edges, which arise due to the interplay of two distinct families of unstable spherical orbits. These solutions provide examples, in which the explicit mechanism for cusp formation can be uncovered.

We examine the shadow cast by a Kerr black hole pierced by a cosmic string. The observable images depend not only on the black hole spin parameter and the angle of inclination, but also on the deficit angle yielded by the cosmic string. The dependence of the observable characteristics of the shadow on the deficit angle is explored. The imprints in the black hole shadow left by the presence of a cosmic string can serve as a method for observational detection of such strings.It's well known that the shadow (or the apparent shape) of a compact relativistic object encodes information about the nature of this object [1]. That is why the apparent shapes of various black holes and other compact objects, such as wormholes and naked singularities, have been intensively studied in the last years. The shadows of the black holes and naked singularities from the Kerr-Newmann family of solutions of the EinsteinMaxwell equations have been thoroughly investigated in [2]- [8]. The shadow of a black hole with a NUT-charge has been obtained in [9]. The black hole shadows in EinsteinMaxwell-dilaton gravity, Chern-Simons modified gravity and braneworld gravity have been examined in [10], [11], [12]. The apparent shape of the Sen black hole has been studied in [13]. The wormhole shadows have been recently investigated in [14], [15].With the advance of technology the experimental observation of the shadows of compact objects is now possible. Experiments that allow such observations include the Event Horizon Telescope [16], which is a system of earth-based telescopes measuring in the (sub)millimeter wavelength, the space-based radio telescopes RadioAstron and Millimetron [17], [18], and the space-based X-ray interferometer MAXIM [19]. In the next few years these missions are expected to reach resolution high enough to observe the shadow of the supermassive compact object at the center of our galaxy or those located at nearby galaxies [18]. The results of these experiments should be compared with the theoretical models. In this way the observations will reject some of the models or will make it possible to distinguish between different types of compact objects [20]. Even more, the mentioned observations can be used for detection of theoretically predicted objects and effects not observed so far. Such a theoretical prediction is the possible existence * E-mail: tintschev@phys.uni-sofia.bg † E-mail: yazad@phys.uni-sofia.bg 1 of cosmic strings. These objects are expected to have formed during phase transitions in the early universe through spontaneous symmetry breakings [21]. It has also been shown that cosmic strings generally form at the end of inflation within the framework of various supersymmetric grand unification theories [22]. A cosmic string makes the space-time around it a conical space-time with a deficit angle δ = 8πGµ/c 4 where µ is the string tension and G and c are the gravitational constant and speed of light, respectively. The deficit angle manifests itself physically by giving rise to interesting phenomena and eff...

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