6709 Hiromiyuki, provisional designation 1989 CD, is a background or Florian asteroid from the inner regions of the asteroid belt, approximately 4 kilometers (2.5 miles) in diameter. It was discovered on 2 February 1989, by Japanese amateur astronomers Masaru Arai and Hiroshi Mori at the Yorii Observatory in Japan.^[1] The possibly elongated S-type asteroid has a rotation period of 6.8 hours.^[5] It was named after the Hiroshi Mori's children, Hiroyuki and Miyuki.^[1]

Hiromiyuki is a non-family asteroid of the main belt's background population when applying the hierarchical clustering method to its proper orbital elements.^[3][4] Based on osculating Keplerian orbital elements, the asteroid has also been classified as a member of the Flora family (402), a giant asteroid family and the largest family of stony asteroids in the main-belt.^[5]

Hiromiyuki orbits the Sun in the inner asteroid belt at a distance of 2.0–2.7 AU once every 3 years and 7 months (1,315 days; semi-major axis of 2.35 AU). Its orbit has an eccentricity of 0.16 and an inclination of 2° with respect to the ecliptic.^[2] The body's observation arc begins with its first observation as 1955 SX at Goethe Link Observatory in September 1955, more than 33 years prior to its official discovery observation at Yorii.^[1]

Hiromiyuki is an assumed S-type asteroid based on its classification into the Flora family.^[5]

This minor planet was named after Hiroyuki (born 1991) and Miyuki Mori (born 1993), son and daughter of the second co-discoverer Hiroshi Mori.^[1] The official naming citation was published by the Minor Planet Center on 6 March 2004 (M.P.C. 51186).^[8]