Embedded defects and symmetry breaking in flipped SU(5)

We explicitly show the analogy between the symmetry breaking scheme for the GUT flipped SU(5) with that of the Weinberg-Salam theory of electroweak interactions. This allows us to construct the embedded defect spectrum of the theory flipped SU(5). We find that the spectrum consists of twelve gauge-equivalent unstable Lepto-quark strings, which are analogous to W-strings in electroweak theory, and another string that is gauge inequivalent to the Lepto-quark strings, which we call the `V-string'. The V-string is analogous to the Z-string of electroweak theory, correspondingly admitting a stable semilocal limit. Using data on the running coupling constants we indicate that in the non-supersymmetric case V-strings can be stable for part of the physically-viable parameter space. Cosmological consequences are briefly discussed.