Instruments in String Chamber Groups

What instruments are in a standard String Quintet, Sextet, Septet and Octet. Is there a standard instrumentation?

QUINTET

''A string quintet is a musical composition for five string players. As an extension to the string quartet (two violins, a viola, and a cello), a string quintet includes a fifth string instrument, usually a second viola (a so-called "viola quintet") or a second cello (a "cello quintet"), or occasionally a double bass'' (Wikipedia, n.d.).

SEXTET

''In classical music, a string sextet is a composition written for six string instruments, or a group of six musicians who perform such a composition. Most string sextets have been written for an ensemble consisting of two violins, two violas, and two cellos'' (Wikipedia, n.d.).

SEPTET

I think the combination of a string septet does not have a standard, since I cannot find it. Just study scores of other composers, who composed for string septet. Maybe you have a sextet with the addition of one double bass. Wikipedia (n.d.) says the following about the septet, which can be useful for you:

''One of the most famous classical septets is the Septet in E-flat major, Op. 20, by Ludwig van Beethoven, composed around 1799–1800, for clarinet, bassoon, horn, violin, viola, cello, and double bass. The popularity of Beethoven's septet made its combination of instruments a standard for subsequent composers'.''

OCTET

A string octet is a piece of music written for eight string instruments, or sometimes the group of eight players. It usually consists of four violins, two violas and two cellos, or four violins, two violas, a cello and a double bass'' (Wikipedia, n.d.).