@misc{W2027761034,
  author = {Curtis G. Callan and Daniel Friedan and Emil J. Martinec and Malcolm J. Perry},
  title = {Strings in background fields},
  year = {1985},
  howpublished = {Nuclear Physics B},
  doi = {https://doi.org/10.1016/0550-3213(85)90506-1}
}