@article{20588,
  abstract     = {In this second paper on our variability survey of central stars of planetary nebulae (CSPNe) using the Zwicky Transient Facility (ZTF), we report 11 long-timescale variables with variability timescales ranging from months to years. We also present preliminary analyses based on spectroscopic and/or photometric follow-up observations for six of them. Among them is NGC 6833, which shows a 980 days periodic variability with strange characteristics: “triangle-shaped” brightening in r, i, and WISE bands but almost coincidental shallow dips in the g-band. The most plausible explanation is a wide binary with the photometric period being the orbital period. Long-period near-sinusoidal variability was detected in two other systems, NGC 6905 and Kn 26, with periods of 700 days and 230 days, respectively, making them additional wide-binary candidates. The latter also shows a short period at 1.18 hr. We then present CTSS 2 and K 3-5, which show brightening and significant reddening over the whole ZTF baseline. A stellar model fit to the optical spectrum of CTSS 2 reveals it to be one of the youngest post-AGB CSPNe known. Both show high-density emission-line cores. We propose these to be late-thermal-pulse candidates, currently evolving towards the AGB phase. We then present recent HST/COS ultraviolet spectroscopy of the known wide-binary candidate LoTr 1, showing that the hot star is a spectroscopic twin of the extremely hot white dwarf in UCAC2 46706450. Similar to this object, LoTr 1 also has a fast rotating wide subgiant companion. We suggest that the long photometric period of 11 yr is the binary orbital period. Finally, we briefly discuss the ZTF light curves of the remaining variables, namely Tan 2, K 3-20, WHTZ 3, Kn J1857+3931, and IPHAS J1927+0814. With these examples, we present the effectiveness of the von Neumann statistics and Pearson Skew-based metric space in searching for long-timescale variables.},
  author       = {Bhattacharjee, Soumyadeep and Reindl, Nicole and Bond, Howard E. and Werner, Klaus and Zeimann, Gregory R. and Jones, David and El-Badry, Kareem and Mackensen, Nina and Chornay, Nicholas and Kulkarni, S. R. and Caiazzo, Ilaria and Van Roestel, Jan and Rodriguez, Antonio C. and Prince, Thomas A. and Rusholme, Ben and Laher, Russ R. and Smith, Roger},
  issn         = {1538-3873},
  journal      = {Publications of the Astronomical Society of the Pacific},
  number       = {10},
  publisher    = {IOP Publishing},
  title        = {{Variability of central stars of planetary nebulae with the Zwicky Transient Facility. II. Long-timescale variables including wide binary and late thermal pulse candidates}},
  doi          = {10.1088/1538-3873/ae051e},
  volume       = {137},
  year         = {2025},
}

@article{21241,
  abstract     = {White dwarfs (WDs) showing transits from orbiting planetary debris provide significant insights into the structure and dynamics of debris disks, which are eventually accreted to produce metal pollution. This is a rare class of objects with only eight published systems. In this work, we perform a systematic search for such systems within 500 pc in the Gaia-eDR3 catalog of WDs using the light curves from the Zwicky Transient Facility (ZTF) and present six new candidates. Our selection process targets the top 1% most photometrically variable sources identified using a combined variability metric from ZTF and Gaia eDR3 photometry, boosted by a metric space we define using von Neumann statistics and Pearson-Skew as a novel discovery tool to identify these systems. This is followed by optical spectroscopic observations of visually selected variables to confirm metal pollution. Four of the six systems show long-timescale photometric variability spanning several months to years, resulting either from long-term evolution of transit activity or dust and debris clouds at wide orbits. Among them, WD J1013–0427 shows an indication of reddening during the long-duration dip. Interpreting this as dust extinction makes it the first system to indicate an abundance of dust grains with radius ≲0.3 μm in the occulting material. The same object also shows metal emission lines that map an optically thick eccentric gas disk orbiting within the star’s Roche limit. For each candidate, we infer the abundances of the photospheric metals and estimate accretion rates. We show that transiting debris systems tend to have higher inferred accretion rates compared to the general population of metal-polluted WDs. Growing the number of these systems will further illuminate such comparative properties in the near future. Separately, we also serendipitously discovered an AM Canis Venaticorum showing a very long-duration outburst—only the fourth such system to be known.},
  author       = {Bhattacharjee, Soumyadeep  and Vanderbosch, Zachary P. and Hollands, Mark A. and Tremblay, Pier-Emmanuel and Xu, Siyi and Guidry, Joseph A. and Hermes, J.J. and Caiazzo, Ilaria and Rodriguez, Antonio C. and van Roestel, Jan and El-Badry, Kareem  and Drake, Andrew J. and Roulston, Benjamin R. and Riddle, Reed and Rusholme, Ben and Groom, Steven L. and Smith, Roger and Toloza, Odette},
  issn         = {1538-3873},
  journal      = {Publications of the Astronomical Society of the Pacific},
  number       = {7},
  publisher    = {IOP Publishing},
  title        = {{A ZTF search for circumstellar debris transits in White Dwarfs: Six new candidates, one with gas disk emission, identified in a novel metric space}},
  doi          = {10.1088/1538-3873/ade0ea},
  volume       = {137},
  year         = {2025},
}