A conjunction (yuti) occurs when two or more planets sit in the same sign of a birth chart, merging their energies. When Sun and Mercury conjoin, the chart blends Sun's the soul, authority and vital life-force, Mercury's intellect, speech and commerce. This is a mixed combination, with 1 benefic and 1 malefic planets involved.
Budhaditya Yoga
This conjunction forms the well-known Budhaditya Yoga, which gives sharp intelligence, eloquence and success in intellectual or administrative work (unless Mercury is too close and combust).
How the Planets Interact
Sun and Mercury are friendly. Friendly planets amplify each other's good results, while hostile planets compete for the same space, creating inner tension that the native must consciously balance. The strongest planet in the group, by dignity, tends to dominate the expression of the conjunction.
Combined Effects
Practically, Sun and Mercury together colour the affairs of whichever house they occupy with a mix of soul (atma), intellect & analysis. Being a mixed combination, results swing between ease and effort depending on placement.For a precise reading, the sign, house and dignity of each planet in your own chart must be examined together.