C.-J. Cheng（NTHU, Institute of Physics)；A.K.H. Kong（NTHU, Institute of Astronomy)；K.L. Li（NCKU, Department of Physics) ；H. Stiele（NTHU, Institute of Astronomy）
The black hole candidate 4U 1755-338 has entered into an X-ray outburst in 2020 April after 24 years of quiescence, and has been in a high/soft X-ray state since then. We observed 4U 1755-338 in 2020 May and July using the 1-m optical telescope at the Lulin Observatory with the SDSS g-, r-, and i-band filters. A variability amplitude of about 0.45 mag is found in all bands, but the 4.4-hr orbital period found in previous outburst is not seen. We speculate that the optical emission of the current outburst is dominated by X-ray irradiation of the accretion disk. We also check the X-ray data taken with NuSTAR (3-79 keV) and there is no evidence for X-ray dips that were seen in previous outburst. In general, the 2020 outburst shows a few different behaviors in both X-ray and optical data compared to the last outburst, indicating that 4U 1755-338 is now in a new type of outburst.