U+1D5D Modifier Letter Small Beta

The Wikipedia has the following information about this codepoint:

In phonetics, vowel roundedness is the amount of rounding in the lips during the articulation of a vowel. It is labialization of a vowel. When a rounded vowel is pronounced, the lips form a circular opening, and unrounded vowels are pronounced with the lips relaxed. In most languages, front vowels tend to be unrounded, and back vowels tend to be rounded. However, some languages, such as French, German and Icelandic, distinguish rounded and unrounded front vowels of the same height (degree of openness), and Vietnamese distinguishes rounded and unrounded back vowels of the same height. Alekano has only unrounded vowels. In the International Phonetic Alphabet vowel chart, rounded vowels are the ones that appear on the right in each pair of vowels. There are also diacritics, U+0339 ◌̹ COMBINING RIGHT HALF RING BELOW and U+031C ◌̜ COMBINING LEFT HALF RING BELOW, to indicate greater and lesser degrees of rounding, respectively. Thus [o̜] has less rounding than cardinal [o], and [o̹] has more (closer to the rounding of cardinal [u]). These diacritics can also be used with unrounded vowels: [ɛ̜] is more spread than cardinal [ɛ], and [ɯ̹] is less spread than cardinal [ɯ].