Research

Symbiotic database

The steadily growing number of known symbiotic stars and candidates has highlighted the need for a comprehensive, up-to-date resource that enables detailed studies of their population and individual properties. To address this, I created the New Online Database of Symbiotic Variables (NODSV), an online catalogue that currently contains more than 1,000 Galactic objects together with nearly 200 systems in external galaxies. The database combines information scattered throughout the literature into a single, homogeneous resource, providing classifications, stellar and orbital parameters, photometric and spectroscopic information, literature references, and interactive visualization tools. Unlike traditional catalogues, NODSV is continuously maintained, allowing newly discovered systems and updated measurements to be incorporated as they become available.

Beyond serving as a catalogue, NODSV forms the foundation of my research on symbiotic stars. It provides the basis for statistical investigations of the Galactic and extragalactic populations, the selection and characterization of new candidates, and the development of machine-learning methods for identifying previously unknown systems in large astronomical surveys. The database has become a widely used resource within the symbiotic-star community and is regularly used by researchers studying interacting binaries.

The first public release of the database was presented in Merc et al. (2019a, 2019b), while its current architecture, expanded content, and the comprehensive statistical overview of the known symbiotic population are described in Merc et al. (2026a).

Analysis of the candidates

A key aspect of my research is the identification and characterization of symbiotic star candidates. To establish a clean and well-defined sample for detailed astrophysical studies, I combine dedicated spectroscopic observations with long-term photometric monitoring, multi-wavelength photometry, astrometric information from the Gaia mission, and archival data from the literature. This comprehensive approach allows me to determine the nature of candidate systems, distinguish genuine symbiotic binaries from other types of interacting or emission-line objects, and investigate their physical properties. More recently, I have also incorporated machine-learning methods into this work, providing new ways of efficiently screening the rapidly growing number of sources available from large astronomical surveys.

The project has evolved from detailed studies of individual symbiotic candidates to the characterization of larger, well-defined samples of interacting binaries. By combining spectroscopic observations with long-term photometric monitoring and archival multi-wavelength data, I have been able to confirm the symbiotic nature of several candidates, investigate unusual systems, and identify objects requiring further follow-up. The resulting samples provide a basis for studying the diversity of symbiotic binaries and their observational properties, as well as for selecting promising targets for more detailed investigations. Results from this research have so far been presented in Merc et al. (2020a, 2021a, 2022a, 2023a, 2023b, 2025, 2026b).

A particularly rewarding aspect of this work has been my collaboration with amateur astronomer Lionel Mulato, with whom I have co-led the scientific analysis and preparation of two studies based on Gaia DR3 emission-line sources. We investigated Wolf–Rayet star candidates and planetary nebula candidates, respectively, and identified several objects with properties suggesting a possible symbiotic nature among these populations (Mulato et al., 2025, 2026). These studies illustrate how collaboration between professional and experienced amateur astronomers, combined with large survey datasets, can produce scientifically valuable results and open new avenues for the discovery and characterization of symbiotic stars.

Search for new symbiotics

The growing volume of data from modern astronomical surveys has opened new opportunities for the systematic discovery of previously unknown symbiotic stars. Building on the New Online Database of Symbiotic Variables and the experience gained from characterizing individual candidates, I develop methods for identifying new symbiotic systems using photometric, spectroscopic, astrometric, and infrared data, complemented by dedicated follow-up observations.

This research has resulted in the discovery, co-discovery, or confirmation of a substantial fraction of the currently known Galactic symbiotic-star population. My systematic searches have identified new systems both within the Milky Way and in external galaxies, including the first Galactic and extragalactic symbiotic systems identified using the Gaia mission (Merc et al., 2020b; Merc, 2022, PhD Thesis). More recently, I have expanded this work to systematic searches within the Gaia DR3 variability catalogue, identifying tens of promising symbiotic candidates and providing a homogeneous framework for exploring the symbiotic-star population in the Gaia data (Merc et al., 2026c, 2026d).

An imporant component of this effort has been the SALT spectroscopic survey of symbiotic candidates, which has led to the identification and characterization of numerous new Galactic symbiotic stars (Merc et al., 2026e). Together with discoveries and confirmations from other observing campaigns, these results demonstrate the effectiveness of combining large survey datasets with targeted spectroscopic follow-up to expand the known population of symbiotic binaries.

This project also benefits from close collaboration between professional and amateur astronomers. Together, we have confirmed the symbiotic nature of newly detected transients (Merc et al., 2021b, 2024), reported the first observed outbursts of several symbiotic stars (Merc et al., 2021c, 2022b, 2023c), and identified new symbiotic systems through coordinated photometric and spectroscopic observations. These collaborations demonstrate the scientific value of long-term community-based monitoring and the important contribution that experienced amateur observers can make to the discovery and follow-up of variable and transient symbiotic stars.