from the Catalina Real-Time Transient Survey [@2009ApJ...696..870D]. KAIT and Catalina data are obtained from unfiltered observations, whose effective color is close to the R-band [@2003PASP..115..844L].

### Host galaxy imaging (NOT) {#HGIMAGE}

To investigate the host galaxy of S2 0109+22, we obtained a deep I-band image at the Nordic Optical Telescope (NOT) on 2015 November 11. In total, 26 exposures, each 150 seconds long, were obtained using the ALFOSC[^11] instrument. After subtracting the bias, flat-fielding and fringe map correction, the images were registered using stars in the field and summed. The resulting image has a total exposure time of 3900 seconds with $FWHM \cong 1.14\arcsec$. The comparison stars in were used to calibrate the field.

### Polarization (NOT)

Polarization observations were carried out using the ALFOSC instrument in the standard linear polarization set-up (lambda/2 retarder followed by calcite) in optical *R*-band. Weekly observations were performed from November 2015 to September 2017 within three observing seasons. In order to determine the zero point of the position angle, polarization standards were observed on a monthly basis. The instrumental polarization was measured observing zero-polarization standard stars, and was found to be negligible. Most of the observations were conducted under good sky condition (seeing $\sim 