# Benchmark ultra-cool dwarfs in widely separated binary systems

## TL;DR

This paper identifies and analyzes widely separated binary systems with ultra-cool dwarfs as companions, providing valuable benchmark objects for studying their physical properties with minimal theoretical dependence.

## Contribution

First identification of a wide ultra-cool dwarf + giant binary system, expanding the set of benchmark objects for astrophysical research.

## Key findings

- Eight ultra-cool dwarfs are confirmed members of wide binary systems.
- SDSS 0832 is a companion to the K3 giant Eta Cancri.
- First wide ultra-cool dwarf + giant binary system discovered.

## Abstract

Ultra-cool dwarfs as wide companions to subgiants, giants, white dwarfs and main sequence stars can be very good benchmark objects, for which we can infer physical properties with minimal reference to theoretical models, through association with the primary stars. We have searched for benchmark ultra-cool dwarfs in widely separated binary systems using SDSS, UKIDSS, and 2MASS. We then estimate spectral types using SDSS spectroscopy and multi-band colors, place constraints on distance, and perform proper motions calculations for all candidates which have sufficient epoch baseline coverage. Analysis of the proper motion and distance constraints show that eight of our ultra-cool dwarfs are members of widely separated binary systems. Another L3.5 dwarf, SDSS 0832, is shown to be a companion to the bright K3 giant Eta Cancri. Such primaries can provide age and metallicity constraints for any companion objects, yielding excellent benchmark objects. This is the first wide ultra-cool dwarf + giant binary system identified.